Shell interpopulation variation and its origin in Pomacea canaliculata (Gastropoda: Ampullariidae) from Southwestern Buenos Aires Province, Argentina

ESTEBENET, ALEJANDRA; MARTÍN, PABLO

JOURNAL OF MOLLUSCAN STUDIES

Oxford University Press

Lugar: Oxford; Año: 2003 vol. 69 p. 301 - 310

0260-1230

Despite its widely recognized conchological variation, studies on shell variability of Pomacea canaliculata are limited to its sexual and ontogenetic components. Here we analyse the interpopulation variation in conchological and somatic traits and sex-related growth patterns of P. canaliculata to determine if it is ecophenotypically or genetically determined. P. canaliculata showed variation in shell shape, shell and body weight and body ash content among populations from three environmentally different sites. Shell shape was also different when snails from the three sources were reared under homogeneous laboratory conditions, indicating a genetic basis for the differences. Shell shapes of laboratory snails differed from their field counterparts, suggesting an environmental influence on shape. Genetic differentiation of shell shape among the studied populations does not seem to be the outcome of adaptation to local conditions or of genetic drift, but is probably a side-effect of the adaptive differentiation in some life-history traits. On the other hand, weight and ash content differences disappeared under homogeneous conditions, suggesting that their variation is mainly ecophenotypic. Variability in shell thickness, body weight and ash content seems to be more related to trophic availability than to water chemistry. In the laboratory, females showed slightly higher growth rates than males, but these inter-sex differences differed among snails from the three sources. However, shell length was not different between sexes in the field, probably due to a deeper effect of food shortage on female growth. The widespread pattern that shells of freshwater snails from different environments are different has been attributed mostly to cumulative environmental effects or to adaptation to local conditions. However, we suggest that different shell shapes could arise as a collateral outcome of genetically different reproductive behaviours and that it would be misleading to study the shell as a trait exposed separately to selective pressures or environmental influences.